Process control systems utilize a variety of field devices to control process parameters. Fluid regulators are commonly distributed throughout process control systems to control the pressures of various fluids (e.g., liquids, gasses, etc.). In general, back pressure fluid regulators modulate or regulate a sensed upstream pressure to maintain upstream process system pressures within acceptable and/or constant pressure limits. For example, a back-pressure fluid regulator typically regulates a pressure of a high pressure fluid source to prevent high pressure fluid from reaching instruments or control devices upstream from the back-pressure regulator.
Some known back pressure fluid regulators include a diaphragm operatively coupled to a valve plug via a valve stem to move the valve plug relative to a valve seat to prevent or allow fluid flow between an inlet and an outlet. For example, back pressure fluid regulators typically regulate an upstream fluid pressure using a pre-set control force or load applied to a first side of the diaphragm to urge the valve plug in a first direction (e.g., a closed direction in which the valve plug sealingly engages the valve seat). A second side of the diaphragm is fluidly coupled to the inlet fluid or upstream pressure to apply a force to urge the diaphragm in a second direction opposite the first direction. Thus, the diaphragm moves the valve plug in response to a difference between the pressure of the fluid at the inlet (i.e., the force applied to the second side of the diaphragm) and the pre-set control force (i.e., the force applied to the first side of the diaphragm) to vary the flow through the regulator to maintain or regulate the desired upstream pressure.
The valve plug and the valve stem are often rigidly coupled together via a fastener so that the valve plug always moves together with the diaphragm. Thus, the valve plug always remains operatively coupled to the diaphragm and the pre-set control or load. As a result, when the valve plug sealingly engages the valve seat, the pre-set force is imparted on a seal (e.g., an elastomeric O-ring) of the valve plug. However, such a connection between the valve plug and the valve stem is disadvantageous because when a control pressure is absent from the sensing chamber during, for example, maintenance or shipment of the fluid regulator, the pre-set load or force can impart a relatively high seat load to the valve plug when the valve plug engages the valve seat. Such relatively high seat load can damage the seal (e.g., an elastomeric seal) of the valve plug, thereby causing the regulator to leak during operation.